In one known type of electrolytic display device, a persistent coloured deposit of an electrochromatic species, present in solution, is formed on a display electrode upon passage of electric current through the solution between a contour electrode and the display electrode. The solution can be colourless or tinted in a colour which contrasts with that of the coloured species.
One class of electrochromic organic substances which have proved of especial interest are the bipyridinium compounds known as viologens. The basis of operation of an electrochromic display device using an electrolyte containing viologen dications, which are colourless and soluble, is the electrochemical reduction of the viologen dication, V.sup.++ and the subsequent reaction with an anion A.sup.-. Upon reduction of the dication at the display electrode, a stable coloured radical cation is formed as follows: EQU V.sup.++ +e.fwdarw.V.sup.+. ( 1)
This combines with the anion to precipitate as a relatively insoluble violet coloured deposit on the display electrode according to the reaction: EQU V.sup.+. +A.sup.- .fwdarw.VA (2)
Some of these viologen based systems, together with a large number of anions are described and claimed in British Patent Specification 1376799 (Philips). In a related paper by Schoot C J, Ponjee J J et al, 1973 J Applied Physics Volume 23, page 64, the particular electrochromic studied was diheptyl-viologen dibromide (N,N'-diheptyl-4,4'-bypyridinium dibromide). On reduction of this viologen, a violet precipitate is formed with the bromide anion at the display electrode. At the anode a reverse reaction occurs. It has been found that the bromide anion corrodes and discolours the metal surface of the display electrode especially when silver, which is a highly preferred metal, is used and repeated reduction/oxidation cycles result in a non-erasible deposit due to recrystallisation of the radical cation deposit.
In U.S. Pat. No. 3,950,077 to R J Jasinski (Texas Instruments), it is suggested that dihydrogen phosphate (H.sub.2 PO.sub.4.sup.-) be substituted for the bromide anion. The use of this anion has solved the recrystallisation and corrosion problems but the speed of the reactions at the electrode is substantially reduced.
In our European published patent application No. 0001912, the use of either or both hypophosphite (H.sub.2 PO.sub.2.sup.-) and phosphite (HPO.sub.3.sup.2-) anions is proposed, optionally in combination with bromide anions. The specification of the application indicates that, mixed with dihydrogen phosphate anions, phosphite or hypophosphite anions lead to substantial improvements in the speeds of the reactions at the display electrode, especially in the erasure of the deposited species, when compared with the use of dihydrogen phosphate alone.
However, in one particular aspect, systems including phosphate, phosphite and hypophosphite anions fall short of the performance of those having bromide anions. A measure of the speed at which a display can be generated (written) is the "take-off" time of a given electrochromic system. Take-off time is closely related to but is not necessarily the same as the classical electrochemical "transition time" which, in the case of viologen, would correspond to the complete depletion of V.sup.++ ions at the cathode surface and to the onset of the further reduction V.sup.+. +e.fwdarw.V.degree. in the material already deposited. Take-off time is a somewhat more general term in that it relates to the observed potential rise irrespective of its cause. Like transition time, however, take-off time is dependent on the density of the applied current, the real surface area of the electrodes and the bulk concentration of electrochromic material in the electrolyte.
Take-off time is measured, in the present case, by noting the time at which the potential across the display electrode/solution interface rises sharply away from the potential associated with deposition of the viologen cation radical. A long take-off time is desirable, for it enables more material to be deposited in a single operation. A long take-off time for a given current density also implies that there is opportunity to shorten the writing time by increasing the current density even though this shortens the take-off time.
The take-off times for electrochromic materials including hypophosphite, phosphite or phosphate anions fall significantly short of the take-off times of systems including bromide anions. In our European published patent application No. 0004548 it is disclosed that modifying the surface of the display electrode in contact with the electrolyte can lead to improvements in take-off times providing the display is operated at a sufficiently high current density. Employing this technique leads to a significant reduction in the attainable write time of the display.
However, even with this improvement, the amount of viologen which can be deposited at acceptable rates, without exceeding the take-off time, is limited to around 3 mCcm.sup.-2. If constant current driving is employed for a longer time period than that needed to pass this magnitude of charge, then irreversible reactions such as the further reduction of the viologen radical cations, V.sup.+., or the evolution of hydrogen ensue. Employing a constant current of density 500 mAcm.sup.-2 for a write time of 4 mS to deposit 2 mCcm.sup.-2 of viologen produces a contrast ratio, against matt silver electrodes, of 2.6:1. This, while quite distinct to the eye, is barely acceptable for some applications. Even with a substantial increase in write time, the maximum attainable contrast with this system is not more than 3.6:1.
The take-off time and, more generally, the speed with which a display can be written is dependent on the concentration of the electrochromic species in solution. The relationship of speed of display writing to concentration and other factors such as diffusion coefficient is discussed in an article entitled "Speed Consideration for Electrochromic Displays" by D J Barclay, C L Bird and D H Martin (Journal of Electronic Materials, Vol. 8, No. 3, 1979, p311). The persistent electrochromic di-alkyl viologens are only sparingly soluble in water and this limits the speed with which a display which operates by electro-reduction of these substances at a cathode can be written.
Speed considerations are particularly important in matrix-addressed displays in which display electrodes are written one row at a time. Using the figure of 4 mS given above, by way of example, for an individual write operation, a display of several hundred rows can only be written in the order of seconds. This is a severe constraint on display performance for certain applications since erasure of the whole display takes less than 150 mS. Alternatively, if write times are limited to, say, 1 mS, unacceptably low contrast ratios result.
The above referenced article mentions, at p318, the possibility of a display system in which a second electroactive species, A.sup.+, is present and observes that such a system would still operate as an electrochromic display providing that the reduction of A.sup.+ took place at a more negative electrode potential than the display reaction and that the reduced species, A.degree., subsequently chemically reduced the electrochromic species to its coloured form.
In an earlier article, two of the above authors had, in fact, proposed such a system for increasing the speed of electrochromic displays ("Increasing Speed of Electrochromic Displays" by D J Barclay and C L Bird, IBM Technical Disclosure Bulletin Vol. 18, No. 10, colour.
This article specifically suggests the addition of zinc (II) or cadmium (II) for this purpose to a heptyl viologen dibromide electrochromic solution. Although, in theory, the electrode reaction potentials and solubilities of these species should increase the speed of writing of a matrix addressed display, in practice the viologen was found to precipitate out of solution as a complex bromide of the metal cation. For this reason a practical display system was not achieved.
To complete the review of the prior art and for the purpose of distinguishing the present invention therefrom, reference should also be made to U.K. Pat. Nos. 1407133 and 1506560, (both ICI). These patents describe electrochromic display devices employing as the electrochromic substances p-cyanophenyl viologens in solution rather than the di-alkyl viologens which are the subject of the prior art discussed above. In addition to p-cyanophenyl viologen the solution contains ferrous ammonium sulphate which constitutes an auxiliary FE.sup.++ /FE.sup.+++ redox system. The purpose of this auxiliary system is to overcome the drawback that p-cyanophenyl viologens, while readily electrochemically reducible, are not electrochemically re-oxidisable. Instead of being electrochemically oxidised, the modified viologen deposit is chemically oxidised by the ferric ions of the auxiliary redox system, in order to erase the display. However, no iron is plated onto the display electrodes and ferrous ions cannot reduce the modified viologen to its coloured form.